Most of chemical, pharmaceutical and coking industries discharge high-strength organic wastewaters with low biodegradability. These effluents are difficult to treat because of containing toxic substances and its high chemical stability. It poses a great risk to the environment and human health once these effluents enter into water body.
Anaerobic wastewater treatment process has obvious advantages in treatment of high-strength organic wastewater due to its high volume load, low energy consumption and even generating energy gas (i.e., CH4). The volume load of anaerobic process can reach several to dozens kilograms of COD/m3 d, which is generally more than ten times of aerobic process. Therefore, anaerobic technology is considered as a suitable method to treat high concentration of organic wastewater.
However, anaerobic process is susceptible to the operational conditions, such as pH, oxidation-reduction potential (ORP) and temperature. Especially, maintaining a neutral pH value is crucial for anaerobic methanogenesis, but pH balance between acidification and methanogenesis is easily destroyed because acidification generates acidity fast while methanogenesis consumes acidity slowly. As a result, the anaerobic performance will decrease when the anaerobic reactor presents acidity.
In order to improve the stability of anaerobic wastewater treatment, it is necessary to create a favorable pH condition for methanogenesis. So far, adding alkaline in the inlet or the anaerobic reactor is frequently used to control acidic pH. However it not only increases the complexity of the operation, but also makes the alkaline concentration in the dosing point much higher than other parts, which may cause serious damage (alkalinity, salinity, anion, etc.) to the anaerobic system.
As a cheap and environmentally friendly reductant, zero-valent iron (ZVI) has received much attention in the field of pollution control in recent years. It was reported that in the anoxic groundwater, ZVI could not only directly decompose organic chlorine as electron donor, but also promote the metabolism of methane-producing bacteria and then speed up the mineralization of organic chlorine. Permeability activity grille (PRB), based on this principle, has been applied in the groundwater treatment and soil remediation. In wastewater treatment, ZVI is mainly used in the preprocessing to improve the biodegradability prior to biotreatment. In our patent application CN200910012293.4, ZVI is put in a single anaerobic reactor, in which releasing ferrous ion compressed the electric double layer to reduce the Zeta potential. As a result, the methanogenesis was enhanced and the anaerobic sludge granulation was accelerated. In addition, ZVI could reduce the ORP in the anaerobic reactor to create a more reductive environment for the growth of methanogenesis.